The process of asphalt compaction for roads, walkways and parking lots has for many years been performed with rolling vehicles having a plurality of cylindrical metal rollers. Compactor work machines having large front and rear drums are a familiar sight in construction zones. In a typical operation, an asphalt paver travels along a prepared bed of gravel, concrete or soil and distributes a layer of hot asphalt in an approximately uniform thickness. Once the asphalt has cooled to an acceptable level and sufficient viscosity, a rolling compactor is passed across the asphalt layer to smooth and compact it. Such work machines are typically relatively heavy to assist in squeezing the asphalt to a hard, uniform surface suitable for road traffic. Depending upon the specific composition, freshly laid asphalt tends to be relatively soft, and workers are therefore forced to wait a significant length of time before the asphalt can withstand the weight of a conventional roller compactor.
One particular problem with soft asphalt is described as a “bow wave” or bulge of asphalt that can form and persist in front of a conventional roller compactor as it travels across the fresh asphalt, pushing the bow wave ahead of the front roller. During a large scale paving operation, the asphalt paver can travel far enough ahead of the compactors that the fresh asphalt has sufficient time to cool and harden before the compactors begin their work. Toward the end of a working day, however, workers are often forced to shut down the pavers well in advance of compaction, allowing the asphalt to cool sufficiently to be compacted, but losing valuable production time.
In recent years, belted asphalt compactors have received increased attention. Compacting the asphalt with a belted work machine offers the advantage of distributing the weight of the machine or the “load” over a larger surface area. Accordingly, the compactor does not sink into the newly laid soft asphalt as readily as a conventional roller compactor. The bow wave phenomenon is much reduced, and less time is required between laying the fresh asphalt and compacting it into a finished product, increasing productivity. Moreover, compaction of softer asphalt is believed to result in tighter compaction and a more aesthetically pleasing surface finish.
A challenge in working softer asphalt with a belted compactor is that turning of the work vehicle is difficult to perform without “scuffing” the asphalt surface as the belt is rotated. The relatively large asphalt contact surface of the belt must slip across the asphalt surface to effect a turn, and in theory, slipping can occur any time the belt rotates relative to the asphalt, risking scuffing.
The actual slip velocity of the belt relative to the asphalt, vehicle weight, and tendency for a particular asphalt—belt interface to scuff are proportional to the risk and degree of scuffing. The relative slip velocity at different positions along a compactor belt—asphalt interface will vary depending upon the particular position relative to a turning axis of the belt. The term “yaw” is used to describe a directional turning of a work vehicle or component such as an asphalt compactor. Thus, the yaw rate, or relative speed at which a belted compactor turns relative to the asphalt surface, is generally proportional to the risk and degree of scuffing.
One known belted compactor design utilizes an elongate belt extending about relatively large front and rear rollers rotating about parallel axes at opposite ends of the compactor. When it is desirable to turn the vehicle, one or both of the rollers are displaced from their axis of rotation by pivoting the same relative to the frame of the work machine, similar to turning a conventional non-belted rolling compactor. Relative displacement between the roller axes by necessity stretches the belt, increasing the belt tension along the outer side of the machine relative to its turning radius, and decreasing the tension along the opposite side. Further still, it is challenging to guide the belt over the rollers during a steering maneuver, as the belt will have a tendency to continue in its straight line direction of travel even as the rollers are turned.
One system for addressing the belt guiding challenge utilizes guide blocks attached to an inside surface of the belt. Rather than single front and rear rollers, the respective rollers are split into two separate rollers having a gap between them that accommodates the guide blocks. As the vehicle is turned, the guide blocks extend into the gap and continuously urge the belt toward its desired orientation. One drawback to such a design is that the vehicle weight or load is concentrated under the split rollers, leaving a strip of less-compacted asphalt under the area of the belt corresponding to the guide blocks.
Another known design uses an articulated dual belt compactor. A front unit and rear unit are coupled together, and a steering apparatus is positioned between the same. When it is desirable to steer the vehicle, the two units are rotated about a steering axis between the tracks, typically extending through the operator platform. In many articulated steering work machines, compactors or otherwise, the rear track briefly moves in a direction opposite the selected turning direction before it begins to follow the front track. This phenomenon is similar to a conventional tow trailer that pivots slightly about an axis between the tow vehicle and trailer prior to following the tow vehicle through a turn. Thus, the rear unit actually performs two steering maneuvers any time the complete machine makes a single steering maneuver. The asphalt has an increased risk of scuffing in areas where the articulated vehicle turns due to this phenomenon.
A further limitation of such a design relates to the capacity, or lack thereof to laterally drive the work vehicle off of the asphalt work surface. With articulation steer, it may be necessary to back the vehicle back and forth several times before it is completely driven off of the asphalt surface, or turn the vehicle about only a very small turning radius.